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LOW FLOW is the WRONG way to go. You want at a minimum, 10X flow but more is better.
In almost 13 yrs I've never had a round tank, always rectangular of many sizes.
The only round "tank" I've used is a muck bucket nursery for raising fry.
In my years here and on the "org" I don't think I've seen even 1% of hobyists using round tanks for their seahorse displays.
I don't consider them to be EXTREMELY fragile, at least not in the way I think you mean, but they are very susceptible to damaging/deadly bacterial and parasitic influences.

Low flow, and very little to zero objects for them to injure themselves on.

Seahorses are extremely fragile, too much flow, can toss them into an object and injure them.

There are some great youtube videos on the subject, and I think the show "Tanked" did an episode on seahorses.

It can be done, and maybe even with less of an investment than a normal reef tank.

Just keep in mind they are extremely fragile, and should be the only inhabitants in the tank.

I'm pretty sure what I saw recommends a round or cylindrical tank vs square/rectangular. This reduces the odds that they will run into a wall and get hurt

Low, medium, and high flow spots are good. Zero objects? You need hitching posts. Seahorses like to grab to things with their tail as they swim around and hitch onto objects like macro algae, branching live rock, gorgonian/other non stinging coral, and more.

Yes youtube has some nice videos on seahorses, just be careful as there is false information out there. Did you just refer someone to see "Tanked" for educational purposes?..

Think you have the wrong idea about fragile. You can pick them up and move them around somewhat quickly with no harm or stress if the horse is use to you. (My female H. Erectus swims to my hand grabbing fingers with her tail now.) Their more fragile in the sense of disease, bacteria, pouch emphysema, gas bubble disease, pathogens, ect. They don't have to be the only inhabitants of the tank. As long as other fish are mellow they shouldn't stress the horses. However this adds a great risk to disease and pathogens the seahorses may have never seen\have no immunity to from the fish. QT the fish and treating them with things like prazipro and what not could help. Also try to get captive bred fish if you must have fish, like a yellow watchman goby or something. I just have snails in my seahorse tank though personally keeping it a species only.

I'm sorry you were only able to see these sites without the benefit of more up to date information. I wish it were not so, but with the internet it is a fact of life on probably any topic you could pick.
Unfortunately, all three pieces were written a long time ago and have many factual pieces of information but also have many facts that are not true. Some of the facts were accepted more than a dozen years ago, but knowledge in the seahorse hobby has evolved very rapidly over the last decade plus years such that many BOOKS written about seahorse were out of date as far as some of the contained information even before they were published. Other facts were just not true right from the get go. For example, stating seahorses preferred cooler temperatues. They don't PREFER cooler temperatures but in captive situations like our tanks, it is VERY advantageous to keep them at lower temperatures to help keep nasty bacteria at bay.
Unfortunately, even the last link there has multiple errors in it as well as other pages on the site, and, IT is from the best internet site available for seahorse keepers.
There just aren't enough knowledgeable people who are capable AND willing to correct so much old information on the site. Much of the information is still valid, but there is so much that experience has shown to be incorrect.
There is another newer seahorse site with more up to date information but it doesn't have near the traffic on the forums there as so many people are unaware of it. You can check out the many articles on it that Tami has amassed in recent years by some knowledgeable people by searching out seahorsetalk fusedjaw.

Yes, what I write is only MY opinion. It is mostly based on my 12 years of seahorse keeping and breeding, but is ALSO made up of how I've interpreted the information gleaned from reading the posts of more knowledgeable hobbyists and breeders on here and on the "org".
I DO at times state that what I write is my opinion, and also at times mention to get other opinions. I DON'T see the need to say this on every post I make.
I feel that it's important to comment on something that posted which is contrary to present acceptance in the hobby so that new hobbyists are not misled.
As for capitalization and use of italics in my writing, it is to emphasize the word much like while talking, we often speak a word in a higher pitch or speak a word with a slightly higher volume to better express what we are saying.
There are a myriad of ways to be successful in seahorse keeping, but there are certain basics that can help the chances of success be greater.

WOW! Maybe someone should look at Rayjay and his experience with seahorses and his successful breeding info.

I don't post because of all the misguided info out there and experience with people wanting something they have no clue about.

Even with the most current info it is personal experience that make for success with these great creatures.

Don't look at the information having feelings but just information (nobody will get offended that way). IT is important to give the correct info at first because we don't know when the person starting the blog will return (it has happened before) and 2 months later they are back wondering why their seahorse in not eating, bloated, or a number of other issues.
ALSO
Quoted::
"Anecdotal info from Rayjay, simply one persons opinion, which means zilch"
your totally WRONG. Rayjay and his info is experience along with trial and error and is one of the MOST (did type one of) with experience in the matter.

That's my story and I am sticking to it....... Now have a wonderful day watching and caring for your ponies. I DO! :-)
Peace out!

I've been seeing a 47G column tank that is 30"H that many people use for their seahorses. But I cannot find a tank like this anywhere. Any suggestions?

Anything could work. As long as its 18" tall you should be okay. I'm sure people have horses in shorter tanks. Taller the better of course. The bigger the tank, the more horses you can get later on too! Keep in mind, past 26" or so gets pretty hard to reach the bottom of the tank without dipping your shoulder in it.